Complex electronic equipment often must operate under a myriad of circumstances and conditions and must perform properly regardless of the environment and regardless of the sequence of operations performed.
Wireless devices, such as cell phones, are one example of a device which must operate under a large variety of conditions in complex environments which are often changing. For this reason devices are laboratory tested to precise scripts which seek to anticipate the various environmental conditions and protocols to which the device will be subjected. However, tests performed in the laboratory, even according to the most rigorous test scripts, often do not test every possible permutation of the actual operating environment. This is so because the number of variable parameters is extremely large and often are dependant upon relative timing between operations or the relative enabling of different protocols in a certain pattern.
Testing, regardless of how imperfect the results, is critically important in the device design stage so that potential problems are discovered and their causes corrected before the design is complete.
One alternative to laboratory script-based testing is to actually take a prototype into the field and test it under various actual operating conditions. Even in such a situation it is unlikely that in any finite period of time the device under test will experience the range of conditions that it will eventually be subjected to during its effective lifecycle.